/// \file       ParamGeneration.cpp
///
/// \brief      Parameter manipulation routines for the Zerocoin cryptographic
///             components.
///
/// \author     Ian Miers, Christina Garman and Matthew Green
/// \date       June 2013
///
/// \copyright  Copyright 2013 Ian Miers, Christina Garman and Matthew Green
/// \license    This project is released under the MIT license.
// Copyright (c) 2017-2019 The PIVX developers

#include "ParamGeneration.h"
#include <string>
#include <cmath>
#include "hash.h"
#include "uint256.h"


namespace libzerocoin {

/// \brief Fill in a set of Zerocoin parameters from a modulus "N".
/// \param N                A trusted RSA modulus
/// \param aux              An optional auxiliary string used in derivation
/// \param securityLevel    A security level
///
/// \throws         std::runtime_error if the process fails
///
/// Fills in a ZC_Params data structure deterministically from
/// a trustworthy RSA modulus "N", which is provided as a CBigNum.
///
/// Note: this routine makes the fundamental assumption that "N"
/// encodes a valid RSA-style modulus of the form "e1*e2" for some
/// unknown safe primes "e1" and "e2". These factors must not
/// be known to any party, or the security of Zerocoin is
/// compromised. The integer "N" must be a MINIMUM of 1023
/// in length, and 3072 bits is strongly recommended.
///

void
CalculateParams(ZerocoinParams &params, CBigNum N, std::string aux, uint32_t securityLevel)
{
    params.initialized = false;
    params.accumulatorParams.initialized = false;

    // Verify that |N| is > 1023 bits.
    uint32_t NLen = N.bitSize();
    if (NLen < 1023) {
        throw std::runtime_error("Modulus must be at least 1023 bits");
    }

    // Verify that "securityLevel" is  at least 80 bits (minimum).
    if (securityLevel < 80) {
        throw std::runtime_error("Security level must be at least 80 bits.");
    }

    // Set the accumulator modulus to "N".
    params.accumulatorParams.accumulatorModulus = N;

    // Calculate the required size of the field "F_p" into which
    // we're embedding the coin commitment group. This may throw an
    // exception if the securityLevel is too large to be supported
    // by the current modulus.
    uint32_t pLen = 0;
    uint32_t qLen = 0;
    calculateGroupParamLengths(NLen - 2, securityLevel, &pLen, &qLen);

    // Calculate candidate parameters ("p", "q") for the coin commitment group
    // using a deterministic process based on "N", the "aux" string, and
    // the dedicated string "COMMITMENTGROUP".
    params.coinCommitmentGroup = deriveIntegerGroupParams(calculateSeed(N, aux, securityLevel, STRING_COMMIT_GROUP),
                                 pLen, qLen);

    // Next, we derive parameters for a second Accumulated Value commitment group.
    // This is a Schnorr group with the specific property that the order of the group
    // must be exactly equal to "q" from the commitment group. We set
    // the modulus of the new group equal to "2q+1" and test to see if this is prime.
    params.serialNumberSoKCommitmentGroup = deriveIntegerGroupFromOrder(params.coinCommitmentGroup.modulus);

    // Calculate the parameters for the internal commitment
    // using the same process.
    params.accumulatorParams.accumulatorPoKCommitmentGroup = deriveIntegerGroupParams(calculateSeed(N, aux, securityLevel, STRING_AIC_GROUP),
            qLen + 300, qLen + 1);

    // Calculate the parameters for the accumulator QRN commitment generators. This isn't really
    // a whole group, just a pair of random generators in QR_N.
    uint32_t resultCtr;
    params.accumulatorParams.accumulatorQRNCommitmentGroup.g = generateIntegerFromSeed(NLen - 1,
            calculateSeed(N, aux, securityLevel, STRING_QRNCOMMIT_GROUPG),
                                             &resultCtr).pow_mod(BN_TWO, N);
    params.accumulatorParams.accumulatorQRNCommitmentGroup.h = generateIntegerFromSeed(NLen - 1,
            calculateSeed(N, aux, securityLevel, STRING_QRNCOMMIT_GROUPH),
                                             &resultCtr).pow_mod(BN_TWO, N);

    // Calculate the accumulator base, which we calculate as "u = C**2 mod N"
    // where C is an arbitrary value. In the unlikely case that "u = 1" we increment
    // "C" and repeat.
    CBigNum constant(ACCUMULATOR_BASE_CONSTANT);
    params.accumulatorParams.accumulatorBase = BN_ONE;
    for (uint32_t count = 0; count < MAX_ACCUMGEN_ATTEMPTS && params.accumulatorParams.accumulatorBase.isOne(); count++) {
        params.accumulatorParams.accumulatorBase = constant.pow_mod(BN_TWO, params.accumulatorParams.accumulatorModulus);
    }

    // Compute the accumulator range. The upper range is the largest possible coin commitment value.
    // The lower range is sqrt(upper range) + 1. Since OpenSSL doesn't have
    // a square root function we use a slightly higher approximation.
    params.accumulatorParams.maxCoinValue = params.coinCommitmentGroup.modulus;
    params.accumulatorParams.minCoinValue = BN_TWO.pow((params.coinCommitmentGroup.modulus.bitSize() / 2) + 3);

    // If all went well, mark params as successfully initialized.
    params.accumulatorParams.initialized = true;

    // If all went well, mark params as successfully initialized.
    params.initialized = true;
}

/// \brief Format a seed string by hashing several values.
/// \param N                A CBigNum
/// \param aux              An auxiliary string
/// \param securityLevel    The security level in bits
/// \param groupName        A group description string
/// \throws         std::runtime_error if the process fails
///
/// Returns the hash of the value.

uint256
calculateGeneratorSeed(uint256 seed, uint256 pSeed, uint256 qSeed, std::string label, uint32_t index, uint32_t count)
{
    CHashWriter hasher(0,0);
    uint256     hash;

    // Compute the hash of:
    // <modulus>||<securitylevel>||<auxString>||groupName
    hasher << seed;
    hasher << std::string("||");
    hasher << pSeed;
    hasher << std::string("||");
    hasher << qSeed;
    hasher << std::string("||");
    hasher << label;
    hasher << std::string("||");
    hasher << index;
    hasher << std::string("||");
    hasher << count;

    return hasher.GetHash();
}

/// \brief Format a seed string by hashing several values.
/// \param N                A CBigNum
/// \param aux              An auxiliary string
/// \param securityLevel    The security level in bits
/// \param groupName        A group description string
/// \throws         std::runtime_error if the process fails
///
/// Returns the hash of the value.

uint256
calculateSeed(CBigNum modulus, std::string auxString, uint32_t securityLevel, std::string groupName)
{
    CHashWriter hasher(0,0);
    uint256     hash;

    // Compute the hash of:
    // <modulus>||<securitylevel>||<auxString>||groupName
    hasher << modulus;
    hasher << std::string("||");
    hasher << securityLevel;
    hasher << std::string("||");
    hasher << auxString;
    hasher << std::string("||");
    hasher << groupName;

    return hasher.GetHash();
}

uint256
calculateHash(uint256 input)
{
    CHashWriter hasher(0,0);

    // Compute the hash of "input"
    hasher << input;

    return hasher.GetHash();
}

/// \brief Calculate field/group parameter sizes based on a security level.
/// \param maxPLen          Maximum size of the field (modulus "p") in bits.
/// \param securityLevel    Required security level in bits (at least 80)
/// \param pLen             Result: length of "p" in bits
/// \param qLen             Result: length of "q" in bits
/// \throws                 std::runtime_error if the process fails
///
/// Calculates the appropriate sizes of "p" and "q" for a prime-order
/// subgroup of order "q" embedded within a field "F_p". The sizes
/// are based on a 'securityLevel' provided in symmetric-equivalent
/// bits. Our choices slightly exceed the specs in FIPS 186-3:
///
/// securityLevel = 80:     pLen = 1024, qLen = 256
/// securityLevel = 112:    pLen = 2048, qLen = 256
/// securityLevel = 128:    qLen = 3072, qLen = 320
///
/// If the length of "p" exceeds the length provided in "maxPLen", or
/// if "securityLevel < 80" this routine throws an exception.

void
calculateGroupParamLengths(uint32_t maxPLen, uint32_t securityLevel,
                           uint32_t *pLen, uint32_t *qLen)
{
    *pLen = *qLen = 0;

    if (securityLevel < 80) {
        throw std::runtime_error("Security level must be at least 80 bits.");
    } else if (securityLevel == 80) {
        *qLen = 256;
        *pLen = 1024;
    } else if (securityLevel <= 112) {
        *qLen = 256;
        *pLen = 2048;
    } else if (securityLevel <= 128) {
        *qLen = 320;
        *pLen = 3072;
    } else {
        throw std::runtime_error("Security level not supported.");
    }

    if (*pLen > maxPLen) {
        throw std::runtime_error("Modulus size is too small for this security level.");
    }
}

/// \brief Deterministically compute a set of group parameters using NIST procedures.
/// \param seedStr  A byte string seeding the process.
/// \param pLen     The desired length of the modulus "p" in bits
/// \param qLen     The desired length of the order "q" in bits
/// \return         An IntegerGroupParams object
///
/// Calculates the description of a group G of prime order "q" embedded within
/// a field "F_p". The input to this routine is in arbitrary seed. It uses the
/// algorithms described in FIPS 186-3 Appendix A.1.2 to calculate
/// primes "p" and "q". It uses the procedure in Appendix A.2.3 to
/// derive two generators "g", "h".

IntegerGroupParams
deriveIntegerGroupParams(uint256 seed, uint32_t pLen, uint32_t qLen)
{
    IntegerGroupParams result;
    CBigNum p;
    CBigNum q;
    uint256 pSeed, qSeed;

    // Calculate "p" and "q" and "domain_parameter_seed" from the
    // "seed" buffer above, using the procedure described in NIST
    // FIPS 186-3, Appendix A.1.2.
    calculateGroupModulusAndOrder(seed, pLen, qLen, &(result.modulus),
                                  &(result.groupOrder), &pSeed, &qSeed);

    // Calculate the generators "g", "h" using the process described in
    // NIST FIPS 186-3, Appendix A.2.3. This algorithm takes ("p", "q",
    // "domain_parameter_seed", "index"). We use "index" value 1
    // to generate "g" and "index" value 2 to generate "h".
    result.g = calculateGroupGenerator(seed, pSeed, qSeed, result.modulus, result.groupOrder, 1);
    result.h = calculateGroupGenerator(seed, pSeed, qSeed, result.modulus, result.groupOrder, 2);

    // Perform some basic tests to make sure we have good parameters
    if ((uint32_t)(result.modulus.bitSize()) < pLen ||          // modulus is pLen bits long
            (uint32_t)(result.groupOrder.bitSize()) < qLen ||       // order is qLen bits long
            !(result.modulus.isPrime()) ||                          // modulus is prime
            !(result.groupOrder.isPrime()) ||                       // order is prime
            !((result.g.pow_mod(result.groupOrder, result.modulus)).isOne()) || // g^order mod modulus = 1
            !((result.h.pow_mod(result.groupOrder, result.modulus)).isOne()) || // h^order mod modulus = 1
            ((result.g.pow_mod(CBigNum(100), result.modulus)).isOne()) ||        // g^100 mod modulus != 1
            ((result.h.pow_mod(CBigNum(100), result.modulus)).isOne()) ||        // h^100 mod modulus != 1
            result.g == result.h ||                                 // g != h
            result.g.isOne()) {                                     // g != 1
        // If any of the above tests fail, throw an exception
        throw std::runtime_error("Group parameters are not valid");
    }

    return result;
}

/// \brief Deterministically compute a  set of group parameters with a specified order.
/// \param groupOrder   The order of the group
/// \return         An IntegerGroupParams object
///
/// Given "q" calculates the description of a group G of prime order "q" embedded within
/// a field "F_p".

IntegerGroupParams
deriveIntegerGroupFromOrder(CBigNum &groupOrder)
{
    IntegerGroupParams result;

    // Set the order to "groupOrder"
    result.groupOrder = groupOrder;

    // Try possible values for "modulus" of the form "groupOrder * 2 * i" where
    // "p" is prime and i is a counter starting at 1.
    for (uint32_t i = 1; i < NUM_SCHNORRGEN_ATTEMPTS; i++) {
        // Set modulus equal to "groupOrder * 2 * i"
        result.modulus = (result.groupOrder * CBigNum(i*2)) + BN_ONE;

        // Test the result for primality
        // TODO: This is a probabilistic routine and thus not the right choice
        if (result.modulus.isPrime(256)) {

            // Success.
            //
            // Calculate the generators "g", "h" using the process described in
            // NIST FIPS 186-3, Appendix A.2.3. This algorithm takes ("p", "q",
            // "domain_parameter_seed", "index"). We use "index" value 1
            // to generate "g" and "index" value 2 to generate "h".
            uint256 seed = calculateSeed(groupOrder, "", 128, "");
            uint256 pSeed = calculateHash(seed);
            uint256 qSeed = calculateHash(pSeed);
            result.g = calculateGroupGenerator(seed, pSeed, qSeed, result.modulus, result.groupOrder, 1);
            result.h = calculateGroupGenerator(seed, pSeed, qSeed, result.modulus, result.groupOrder, 2);

            // Perform some basic tests to make sure we have good parameters
            if (!(result.modulus.isPrime()) ||                          // modulus is prime
                    !(result.groupOrder.isPrime()) ||                       // order is prime
                    !((result.g.pow_mod(result.groupOrder, result.modulus)).isOne()) || // g^order mod modulus = 1
                    !((result.h.pow_mod(result.groupOrder, result.modulus)).isOne()) || // h^order mod modulus = 1
                    ((result.g.pow_mod(CBigNum(100), result.modulus)).isOne()) ||        // g^100 mod modulus != 1
                    ((result.h.pow_mod(CBigNum(100), result.modulus)).isOne()) ||        // h^100 mod modulus != 1
                    result.g == result.h ||                                 // g != h
                    result.g.isOne()) {                                     // g != 1
                // If any of the above tests fail, throw an exception
                throw std::runtime_error("Group parameters are not valid");
            }

            return result;
        }
    }

    // If we reached this point group generation has failed. Throw an exception.
    throw std::runtime_error("Too many attempts to generate Schnorr group.");
}

/// \brief Deterministically compute a group description using NIST procedures.
/// \param seed                         A byte string seeding the process.
/// \param pLen                         The desired length of the modulus "p" in bits
/// \param qLen                         The desired length of the order "q" in bits
/// \param resultModulus                A value "p" describing a finite field "F_p"
/// \param resultGroupOrder             A value "q" describing the order of a subgroup
/// \param resultDomainParameterSeed    A resulting seed for use in later calculations.
///
/// Calculates the description of a group G of prime order "q" embedded within
/// a field "F_p". The input to this routine is in arbitrary seed. It uses the
/// algorithms described in FIPS 186-3 Appendix A.1.2 to calculate
/// primes "p" and "q".

void
calculateGroupModulusAndOrder(uint256 seed, uint32_t pLen, uint32_t qLen,
                              CBigNum *resultModulus, CBigNum *resultGroupOrder,
                              uint256 *resultPseed, uint256 *resultQseed)
{
    // Verify that the seed length is >= qLen
    if (qLen > (sizeof(seed)) * 8) {
        // TODO: The use of 256-bit seeds limits us to 256-bit group orders. We should probably change this.
        // throw std::runtime_error("Seed is too short to support the required security level.");
    }

#ifdef ZEROCOIN_DEBUG
    cout << "calculateGroupModulusAndOrder: pLen = " << pLen << endl;
#endif

    // Generate a random prime for the group order.
    // This may throw an exception, which we'll pass upwards.
    // Result is the value "resultGroupOrder", "qseed" and "qgen_counter".
    uint256     qseed;
    uint32_t    qgen_counter;
    *resultGroupOrder = generateRandomPrime(qLen, seed, &qseed, &qgen_counter);

    // Using pLen / 2 + 1 as the length and qseed as the input_seed, use the random prime
    // routine to obtain p0 , pseed, and pgen_counter. We pass exceptions upward.
    uint32_t    p0len = ceil((pLen / 2.0) + 1);
    uint256     pseed;
    uint32_t    pgen_counter;
    CBigNum p0 = generateRandomPrime(p0len, qseed, &pseed, &pgen_counter);

    // Set x = 0, old_counter = pgen_counter
    uint32_t    old_counter = pgen_counter;

    // Generate a random integer "x" of pLen bits
    uint32_t iterations;
    CBigNum x = generateIntegerFromSeed(pLen, pseed, &iterations);
    pseed += (iterations + 1);

    // Set x = 2^{pLen-1} + (x mod 2^{pLen-1}).
    CBigNum powerOfTwo = BN_TWO.pow(pLen-1);
    x = powerOfTwo + (x % powerOfTwo);

    // t = x / (2 * resultGroupOrder * p0).
    // TODO: we don't have a ceiling function
    CBigNum t = x / (BN_TWO * (*resultGroupOrder) * p0);

    // Now loop until we find a valid prime "p" or we fail due to
    // pgen_counter exceeding ((4*pLen) + old_counter).
    for ( ; pgen_counter <= ((4*pLen) + old_counter) ; pgen_counter++) {
        // If (2 * t * resultGroupOrder * p0 + 1) > 2^{pLen}, then
        // t = 2^{pLen-1} / (2 * resultGroupOrder * p0)
        powerOfTwo = BN_TWO.pow(pLen);
        CBigNum prod = (BN_TWO * t * (*resultGroupOrder) * p0) + BN_ONE;
        if (prod > powerOfTwo) {
            // TODO: implement a ceil function
            t = BN_TWO.pow(pLen-1) / (BN_TWO * (*resultGroupOrder) * p0);
        }

        // Compute a candidate prime resultModulus = 2tqp0 + 1.
        *resultModulus = (BN_TWO * t * (*resultGroupOrder) * p0) + BN_ONE;

        // Verify that resultModulus is prime. First generate a pseudorandom integer "a".
        CBigNum a = generateIntegerFromSeed(pLen, pseed, &iterations);
        pseed += iterations + 1;

        // Set a = 2 + (a mod (resultModulus - 3)).
        a = BN_TWO + (a % ((*resultModulus) - BN_THREE));

        // Set z = a^{2 * t * resultGroupOrder} mod resultModulus
        CBigNum z = a.pow_mod(BN_TWO * t * (*resultGroupOrder), (*resultModulus));

        // If GCD(z-1, resultModulus) == 1 AND (z^{p0} mod resultModulus == 1)
        // then we have found our result. Return.
        if ((resultModulus->gcd(z - BN_ONE)).isOne() &&
                (z.pow_mod(p0, (*resultModulus))).isOne()) {
            // Success! Return the seeds and primes.
            *resultPseed = pseed;
            *resultQseed = qseed;
            return;
        }

        // This prime did not work out. Increment "t" and try again.
        t = t + BN_ONE;
    } // loop continues until pgen_counter exceeds a limit

    // We reach this point only if we exceeded our maximum iteration count.
    // Throw an exception.
    throw std::runtime_error("Unable to generate a prime modulus for the group");
}

/// \brief Deterministically compute a generator for a given group.
/// \param seed                         A first seed for the process.
/// \param pSeed                        A second seed for the process.
/// \param qSeed                        A third seed for the process.
/// \param modulus                      Proposed prime modulus for the field.
/// \param groupOrder                   Proposed order of the group.
/// \param index                        Index value, selects which generator you're building.
/// \return                             The resulting generator.
/// \throws                             A std::runtime_error if error.
///
/// Generates a random group generator deterministically as a function of (seed,pSeed,qSeed)
/// Uses the algorithm described in FIPS 186-3 Appendix A.2.3.

CBigNum
calculateGroupGenerator(uint256 seed, uint256 pSeed, uint256 qSeed, CBigNum modulus, CBigNum groupOrder, uint32_t index)
{
    CBigNum result;

    // Verify that 0 <= index < 256
    if (index > 255) {
        throw std::runtime_error("Invalid index for group generation");
    }

    // Compute e = (modulus - 1) / groupOrder
    CBigNum e = (modulus - BN_ONE) / groupOrder;

    // Loop until we find a generator
    for (uint32_t count = 1; count < MAX_GENERATOR_ATTEMPTS; count++) {
        // hash = Hash(seed || pSeed || qSeed || "ggen" || index || count
        uint256 hash = calculateGeneratorSeed(seed, pSeed, qSeed, "ggen", index, count);
        CBigNum W(hash);

        // Compute result = W^e mod p
        result = W.pow_mod(e, modulus);

        // If result > 1, we have a generator
        if (result > 1) {
            return result;
        }
    }

    // We only get here if we failed to find a generator
    throw std::runtime_error("Unable to find a generator, too many attempts");
}

/// \brief Deterministically compute a random prime number.
/// \param primeBitLen                  Desired bit length of the prime.
/// \param in_seed                      Input seed for the process.
/// \param out_seed                     Result: output seed from the process.
/// \param prime_gen_counter            Result: number of iterations required.
/// \return                             The resulting prime number.
/// \throws                             A std::runtime_error if error.
///
/// Generates a random prime number of primeBitLen bits from a given input
/// seed. Uses the Shawe-Taylor algorithm as described in FIPS 186-3
/// Appendix C.6. This is a recursive function.

CBigNum
generateRandomPrime(uint32_t primeBitLen, uint256 in_seed, uint256 *out_seed,
                    uint32_t *prime_gen_counter)
{
    // Verify that primeBitLen is not too small
    if (primeBitLen < 2) {
        throw std::runtime_error("Prime length is too short");
    }

    // If primeBitLen < 33 bits, perform the base case.
    if (primeBitLen < 33) {
        CBigNum result(0);

        // Set prime_seed = in_seed, prime_gen_counter = 0.
        uint256     prime_seed = in_seed;
        (*prime_gen_counter) = 0;

        // Loop up to "4 * primeBitLen" iterations.
        while ((*prime_gen_counter) < (4 * primeBitLen)) {

            // Generate a pseudorandom integer "c" of length primeBitLength bits
            uint32_t iteration_count;
            CBigNum c = generateIntegerFromSeed(primeBitLen, prime_seed, &iteration_count);
#ifdef ZEROCOIN_DEBUG
            cout << "generateRandomPrime: primeBitLen = " << primeBitLen << endl;
            cout << "Generated c = " << c << endl;
#endif

            prime_seed += (iteration_count + 1);
            (*prime_gen_counter)++;

            // Set "intc" to be the least odd integer >= "c" we just generated
            uint32_t intc = c.getulong();
            intc = (2 * floor(intc / 2.0)) + 1;
#ifdef ZEROCOIN_DEBUG
            cout << "Should be odd. c = " << intc << endl;
            cout << "The big num is: c = " << c << endl;
#endif

            // Perform trial division on this (relatively small) integer to determine if "intc"
            // is prime. If so, return success.
            if (primalityTestByTrialDivision(intc)) {
                // Return "intc" converted back into a CBigNum and "prime_seed". We also updated
                // the variable "prime_gen_counter" in previous statements.
                result = intc;
                *out_seed = prime_seed;

                // Success
                return result;
            }
        } // while()

        // If we reached this point there was an error finding a candidate prime
        // so throw an exception.
        throw std::runtime_error("Unable to find prime in Shawe-Taylor algorithm");

        // END OF BASE CASE
    }
    // If primeBitLen >= 33 bits, perform the recursive case.
    else {
        // Recurse to find a new random prime of roughly half the size
        uint32_t newLength = ceil((double)primeBitLen / 2.0) + 1;
        CBigNum c0 = generateRandomPrime(newLength, in_seed, out_seed, prime_gen_counter);

        // Generate a random integer "x" of primeBitLen bits using the output
        // of the previous call.
        uint32_t numIterations;
        CBigNum x = generateIntegerFromSeed(primeBitLen, *out_seed, &numIterations);
        (*out_seed) += numIterations + 1;

        // Compute "t" = x / (2 * c0)
        // TODO no Ceiling call
        CBigNum t = x / (BN_TWO * c0);

        // Repeat the following procedure until we find a prime (or time out)
        for (uint32_t testNum = 0; testNum < MAX_PRIMEGEN_ATTEMPTS; testNum++) {

            // If ((2 * t * c0) + 1 > 2^{primeBitLen}),
            // then t = (2^{primeBitLen} - 1) / (2 * c0)
            if ((BN_TWO * t * c0) > (BN_TWO.pow(CBigNum(primeBitLen)))) {
                t = ((BN_TWO.pow(CBigNum(primeBitLen))) - BN_ONE) / (BN_TWO * c0);
            }

            // Set c = (2 * t * c0) + 1
            CBigNum c = (BN_TWO * t * c0) + BN_ONE;

            // Increment prime_gen_counter
            (*prime_gen_counter)++;

            // Test "c" for primality as follows:
            // 1. First pick an integer "a" in between 2 and (c - 2)
            CBigNum a = generateIntegerFromSeed(c.bitSize(), (*out_seed), &numIterations);
            a = BN_TWO + (a % (c - BN_THREE));
            (*out_seed) += (numIterations + 1);

            // 2. Compute "z" = a^{2*t} mod c
            CBigNum z = a.pow_mod(BN_TWO * t, c);

            // 3. Check if "c" is prime.
            //    Specifically, verify that gcd((z-1), c) == 1 AND (z^c0 mod c) == 1
            // If so we return "c" as our result.
            if (c.gcd(z - BN_ONE).isOne() && z.pow_mod(c0, c).isOne()) {
                // Return "c", out_seed and prime_gen_counter
                // (the latter two of which were already updated)
                return c;
            }

            // 4. If the test did not succeed, increment "t" and loop
            t = t + BN_ONE;
        } // end of test loop
    }

    // We only reach this point if the test loop has iterated MAX_PRIMEGEN_ATTEMPTS
    // and failed to identify a valid prime. Throw an exception.
    throw std::runtime_error("Unable to generate random prime (too many tests)");
}

CBigNum
generateIntegerFromSeed(uint32_t numBits, uint256 seed, uint32_t *numIterations)
{
    CBigNum      result(0);
    uint32_t    iterations = ceil((double)numBits / (double)HASH_OUTPUT_BITS);

#ifdef ZEROCOIN_DEBUG
    cout << "numBits = " << numBits << endl;
    cout << "iterations = " << iterations << endl;
#endif

    // Loop "iterations" times filling up the value "result" with random bits
    for (uint32_t count = 0; count < iterations; count++) {
        // result += ( H(pseed + count) * 2^{count * p0len} )
        result += CBigNum(calculateHash(seed + count)) * BN_TWO.pow(count * HASH_OUTPUT_BITS);
    }

    result = BN_TWO.pow(numBits - 1) + (result % (BN_TWO.pow(numBits - 1)));

    // Return the number of iterations and the result
    *numIterations = iterations;
    return result;
}

/// \brief Determines whether a uint32_t is a prime through trial division.
/// \param candidate       Candidate to test.
/// \return                true if the value is prime, false otherwise
///
/// Performs trial division to determine whether a uint32_t is prime.

bool
primalityTestByTrialDivision(uint32_t candidate)
{
    // TODO: HACK HACK WRONG WRONG
    CBigNum canBignum(candidate);

    return canBignum.isPrime();
}

} // namespace libzerocoin
